Sound masking is the addition of natural or artificial sound (such as white noise) into an environment to cover up unwanted sound. This is in contrast to the technique of active noise control. Sound masking reduces or eliminates awareness of pre-existing sounds in a given environment and can make the environment more comfortable. For example, devices are commercially available for being installed in a room in order to mask sounds that otherwise might interfere with a person's working or sleeping in the room.
It is known in the art that not the peak sound-level, but rather the peak-to-baseline sound-level is related to the number of awakenings caused by the sounds to the patient's sleep. By adding a masking sound, therefore, the threshold for being awakened from sleep is raised, resulting in a more comfortable sleep environment. See, e.g., Stanchina, M., Abu-Hijleh, M., Chaudhry, B. K., Carlisle, C. C., Millman, R. P. (2005), “The influence of white noise on sleep in subjects exposed to ICU noise”, Sleep Medicine 6(5): 423-428, for a discussion of the relationship between peak-to-baseline sound-level and threshold within the context of experiments conducted at an intensive-care unit of a hospital.
Sound masking devices are commercially available that produce stationary acoustic noise in a relatively wide frequency band to reduce the chance that a user will get awakened during his/her sleep as a result of ambient sounds. In some of these devices, a microphone is used to capture the potentially disturbing sound for subjecting the potentially disturbing sound to an analysis in order to adjust the masking sound to the level of the intensity of the disturbing sound and to the spectral characteristics of the disturbing sound.
The commercially available sound masking devices typically use a single loudspeaker to reproduce a sound in a relatively wide frequency-band, e.g., white noise. Some of the commercially available products come with a headphone connection, so that the masking sound does not disturb nearby persons in operational use of the product. However, the sound reproduced over the headphones is often only a duplication of the single channel.